The microbolometer (or bolometer) operates on the principle that the electrical resistance of the microbolometer material changes with respect to the microbolometer temperature, which in turn changes in response to the quantity of absorbed incident infrared radiation. These characteristics may be exploited to measure incident infrared radiation on the microbolometer by sensing the resulting change in its resistance.
Modern microbolometer structures are typically fabricated on monolithic silicon substrates to form an array of microbolometers, with each microbolometer functioning as a pixel to produce a two-dimensional image. The change in resistance of each microbolometer is translated into a time-multiplexed electrical signal by circuitry known as the read out integrated circuit (ROIC), which is typically formed within the silicon substrate upon which the microbolometer array is fabricated. The combination of the ROIC and the microbolometer array is commonly known as a microbolometer focal plane array (FPA) or microbolometer infrared FPA.
The microbolometer is generally thermally isolated from its supporting substrate or surroundings by forming an air-bridge structure (microbolometer bridge or microbridge) to allow the absorbed incident infrared radiation to generate a temperature change in the microbolometer material. For example, a conventional microbolometer array structure may be formed as a two-dimensional array of closely spaced air-bridge structures that are coated with a temperature sensitive resistive material, such as vanadium oxide, that absorbs infrared radiation.
The conventional air-bridge structure (which may refer to and be implemented as a vacuum-gap structure) provides good thermal isolation between the microbolometer and the silicon substrate and also forms a resonant cavity structure for improved infrared absorption, with the silicon substrate typically coated with a reflective material to reflect the infrared radiation back to the microbolometer. Thus, the air-gap thickness (i.e., the approximate distance between the reflective material and the air-bridge structure) may determine to a substantial degree the infrared absorption characteristics and spectral response of the microbolometers.
A drawback of a conventional microbolometer FPA is that the air-gap thickness is fixed during the manufacturing process and, consequently, the spectral response of the microbolometer array is limited by the resonant cavity structure formed during the manufacturing process. Another drawback, for example, of a conventional microbolometer FPA is that a mechanical shutter is typically required to calibrate the microbolometer FPA, with the mechanical shutter often being relatively difficult to manufacture, with certain labor intensive and expensive manufacturing processes. Furthermore, the mechanical shutter may be viewed as being a slow mechanism relative to the electronics and microbolometer FPA capabilities, with the mechanical shutter typically having a number of mechanical components that may degrade the reliability, may increase power consumption, and possibly reduce the performance of an infrared camera incorporating the microbolometer FPA.
As a result, there is a need for improved techniques for detecting infrared radiation with microbolometer FPAs and, for example, calibrating the microbolometer FPA.